Ecological Concern; Diagnostic Challenge: Secondary Rodenticide Toxicity in Predatory Birds

In late April, 2012, an American kestrel found on the railway line was submitted by the local municipality to Western/Northern Canadian Cooperative Wildlife Health Centre (CCWHC) laboratory in Saskatoon. There was concern that the bird may have been poisoned by a second generation anticoagulant rodenticide, chlorophacinone, that the municipality had been using for rodent control along the tracks. The bird was thin with no visible fat stores and appeared to have died from starvation. There was no evidence of hemorrhage or trauma except for small, matching erosions on the leading edge of the wings at the level of the primary feathers. Microscopically, there were no significant lesions. In order to address the concerns of the municipality, a portion of the liver was submitted for an anticoagulant rodenticide screen.

Brodifacoum (a second generation anticoagulant rodenticide) was detected and interpreted as significant by the toxicologist. Second generation anticoagulants are related to warfarin, a blood-thinning compound often used in human medicine. Animals die from blood loss and/or hemorrhages in tissues when they receive a toxic dose of the compound. The diagnostic challenge was whether brodifacoum contributed to the death of the emaciated kestrel when there was no evidence of hemorrhage, the hallmark of anticoagulant toxicity. The only conclusion that could be made in this case was that the kestrel was exposed to brodifacoum.  This highlights the need for public education regarding the bioavailability of these chemicals to non-target birds, animals and human beings.

A recent article published in Nature News, Killing rats is killing birds, highlights the evolving understanding of the effect of second generation anticoagulant rodenticides on the health of predatory birds. A large proportion of great-horned owls and red-tailed hawks that were sampled in the Vancouver area had second-generation anticoagulant rodenticide residues in their livers. An estimated 11% of the great horned owls sampled from across Canada were thought to be at risk for being directly killed by second generation anticoagulant rodenticides (Thomas PJ, et. al.).  The toxicity and persistence of these compounds has prompted a change in regulation (effective January 01, 2013) with restricted outdoor use, however there is some skepticism about whether changing rodenticide usage regulations will make a difference if public awareness is not improved.

The loss of these birds of prey on the landscape not only has a negative effect on the biodiversity and ecology of an area, but also eliminates the natural pest control service that these animals provide.

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1 Response

  1. 2013-06-04

    […] Rat poisons are widely used to suppress rats and mice, and they can be quite effective at suppressing these and many other rodent populations. Unfortunately, rat poisons are not only affecting unwanted rats.  Accruing toxicity data from several countries are showing that these poisons are getting out into the larger ecosystem, negatively impacting both mammals and birds. […]

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